1. Field of Invention
The present invention pertains to the art of cooking appliances and, more particularly, to a microwave energy delivery system including a toroidal waveguide which efficiently delivers a substantially uniform microwave energy field into a cooking chamber.
2. Discussion of Prior Art
Cooking appliances utilizing directed microwave energy fields to cook food items have existed for some time. In general, a cooking process is performed by heating the food item by directing a standing microwave energy field into an oven cavity such that the microwave energy field reflects about the oven cavity and impinges upon the food item. As the microwave energy field impinges upon the food item, the field is converted into heat through two mechanisms. The first, ionic heating is caused by the liner acceleration of ions, generally in the form of salts present within the food item. The second is the molecular excitation of polar molecules, primarily water, present within the food item. However, the nature of the standing waves results in localized areas of high and low energy which cause the food to cook unevenly. This is especially true in larger ovens where the size of the cavity requires a more uniform energy distribution in order to properly cook the food. To attain an even or uniform energy distribution, the microwave energy must be introduced into the oven cavity in a manner which creates a constructive standing wave front which will propagate about the oven cavity in a random fashion.
Various methods of directing microwaves into cooking chambers to minimize hot and cold areas resulting from the existence of high and low energy fields have been proposed in the prior art. These methods range from altering the pattern of the standing waves by varying the frequency of the microwave energy field, to incorporating a stationary mode stirrer which simulates change in the geometric space of the cooking chamber. Methods of changing the wave pattern have also included the incorporation of a rotating blade stirrer which functions to deflect microwave energy into a cooking cavity in various patterns. Traditionally, stirrers have been located in various points in the microwave feed system, ranging from adjacent to a microwave energy source to a position within the cooking chamber itself. Some stirrers include various openings which are provided to disperse standing waves, and others have various surface configurations designed to reflect the standing waves. Stirrers are either driven by a motor or by air currents supplied by a blower. In any event, all of these methods share a common theme, i.e., to reflect and/or deflect the microwave energy into a cooking cavity such that a uniform distribution of standing wave patterns can be achieved.
Other methods of controlling the standing waves include modifying the structure of the waveguide itself. The prior art provides examples of waveguides shown as cylinders, square boxes, and a variety of other configurations designed to cause the standing waves to interfere with one another in a manner which results in a randomized wave front such that a maximum energy field is directed into the cooking chamber. Other designs include matching the dimensions of the waveguide to the wavelengths of the standing wave pattern. However, these designs, while effective to a point, have failed to adequately address the problem of energy loss due to energy absorption on the waveguide surface.
As the desire to increase the sizes of oven cavities has risen, and microwave technology has been combined into conventional or convection ovens, the uniform distribution of the standing waves has become of even greater concern. For this reason, manufacturers have modified their designs to include multiple magnetrons, multiple stirrers, and motor driven, variable speed stirrers, all of which are intended to create a random wave pattern thought to be of a more uniform character. Still other designs include structure for rotating or moving food within the cooking chamber. Ovens employing this method, position the food on a platter which is rotated through the standing wave patterns such that the food is more uniformly exposed to the microwaves.
While these methods are fine for smaller ovens, they are hardly practical for larger, conventional, ovens where space is more of a concern. Certainly, in an age where energy consumption is of particular concern, the need for an energy efficient cooking appliance is desired. Based on the above, there exists a need for a microwave delivery system which will direct a uniform standing wave pattern into a cooking chamber in a manner that minimizes energy losses within a waveguide, yet provides a uniform, maximum energy field to the cooking chamber.
The present invention is directed to a microwave cooking appliance including a toroidal-shaped waveguide preferably having a ring diameter equal to twice the standing wavelength, and a cross-sectional diameter equal to one-half the standing wavelength. The design of the waveguide causes a standing wave to impinge upon an inner surface of the waveguide at points of zero energy such that energy absorbed by the waveguide is minimized. Additionally, the design of the ring diameter causes constructive interferences within the waveguide, thereby a high energy node about the circumference of the waveguide. Furthermore, a plurality of cavity excitation ports are arranged about the bottom portion of the waveguide.
In accordance with a preferred embodiment, the microwave delivery system of the present invention further includes a mode stirrer having a plurality of openings evenly spaced about the periphery of the stirrer. Specifically, when rotated, the openings operate as shutters. As the openings align with the cavity excitation ports, passages are created to allow the microwave energy field into the cooking chamber. The operation of the stirrer creates a uniform pattern of microwave energy to be directed onto a food item placed within the cooking chamber.